Currently i have turned on POP-before-SMTP, but some customers who use old ms outlook versions can´t use that.
Why not - they just need to check their mail before sending, that is the only requirement. If their program is that old that something makes it not work, tell them they need to get with the program and update it or they will not be able to send mail. Like Windows 3.11, tell them your software won't run on old antiquated stuff.
While you're at it, tell them Outlook is the worst program available, it is one of the reasons there is so much of a virus problem out there, it helps perpetrate viruses. Tell them to get Eudora and stop spreading viruses like wildfire.
Also, Pop-before-relay does not prevent you from being spammed, it prevents *others* from not being spammed. Spammers just need a relay host to do their dirty work, it is not there to save your users from SPAM, it is to save the world from it.
Jale